Undergravel fishtank filter



FIG. 1 is a top front perspective view of an undergravel fishtank filter showing my new design;

FIG. 2 is a top plan view thereof;

FIG. 3 is a front elevational view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a right side elevational view thereof, the left side being a mirror image; and

FIG. 6 is a top, rear perspective view thereof. 

The ornamental design for an undergravel fishtank filter, as shown and described. 